This invention relates to magnetic recording media, and more particularly to an electroless deposition process for producing magnetic recording films having substantially square hysteresis characteristics and magnetic coercivity values that may be adjusted in the plating bath to lie in the range of 300 to 1,000 oersteds.
The electroless deposition process of the invention is particularly useful for the manufacture of large capacity digital computer memory systems, such as magnetic memory discs. The reading and writing of information on the surface of a memory disc is usually accomplished by a magnetic transducer in a flying head which supports the transducer at a carefully controlled and very close spacing to the surface of the disc. During writing operations, the magnetic transducer converts the electrical input current pulses into a bidirectional magnetic field which then locally changes the direction of the magnetic flux in the film of magnetic medium on the disc surface. During the reading operation, the flux reversals in the magnetic medium are converted by the transducer into an electrical voltage output pattern.
It is common practice to manufacture magnetic memory devices, such as memory discs, on an aluminum substrate. When the magnetic media is a metallic film, the aluminum substrate is first coated with a hard non-magnetic layer upon which is deposited the magnetic recording media layer, generally a cobalt-phosphorus or cobalt-nickel-phosphorus alloy. It is also common practice to overcoat the magnetic layer to protect it from oxidation and physical damage. Although electroplating has occasionally been employed to form the layers in the magnetic structure, electroless deposition is usually preferred because it lends itself to batch processing of many memory discs at one time in a single plating tank.
In the past, the general practice has been to deposit the magnetic thin films at thicknesses greater than 5 or even 10 microinches since the voltage output is related to the thickness of the magnetic layer and since it was desired to obtain higher output voltages from the media. However, it has been found that greatly improved performance is obtainable with much thinner magnetic films in order to record the magnetic information at a greater density along the track of the flying head. In order to obtain a high packing density such as 5,000 bits per inch, or more, metallic films of 5 microinches in thickness or less are necessary. However, because the reduced thickness of the film will reduce the signal output from the magnetic medium, the thin film must possess very good magnetic hysteresis loop qualities to prevent further degradation of the output signal. The magnetic hysteresis loop characteristics which are important include the magnetic remanence, the coercive force, and the loop shape, which is often referred to as the squareness.
It is most desirable that the sides of the hysteresis loop be made as vertical as possible in order to reduce the time required for a recording transducer to switch the direction of the magnetic flux in the film. If the transducer time is negligible compared with the movement of the magnetic media during writing, the slope of the loop sides would not be important. However, in high-speed recording where the transducer is required to switch its magnetic field at a very high rate, then sloping sides of the hysteresis loop causes the magnetic flux change to be smeared by the motion of the media, thereby resulting in a readout signal that is quite low compared with that from a media having a hysteresis loop with nearly vertical sides. It can be seen, therefore, that the design of a magnetic memory system represents a compromise between information packing density, voltage output signal, and writing currents as determined by the hysteresis loop properties which govern these parameters.
The invention described and claimed herein provides an improved magnetic recording media having a high remanent magnetization and good hysteresis loop characteristics so that the highest values of coercive force can be employed together with good resolution. Specifically, the invention provides magnetic films in which the remanent magnetization is higher than 10,000 gauss, and the coercive force can be selected to lie between about 300 and 1,000 oersteds while the slope of the sides of the hysteresis loop are substantially vertical even though the magnetic film thickness is less than 5 microinches.
Broadly, the plating process used in my invention includes the following prior art steps: First, a cleaned aluminum substrate is plated with a very thin layer of zinc and then placed in an electroless nickel bath which is so formulated that a high-phosphorus content, non-magnetic nickel is deposited to a thickness of at least 60 microinches. The nickel-coated substrate is then applied to an electroless bath and a cobalt-phosphorus magnetic layer is deposited on the nickel surface. The above broadly outlined process has been found satisfactory for producing thick magnetic layers for low packing density recording. However, the above process has, in the past, presented many problems when magnetic coatings of a thickness approximately 5 microinches or less are required. The most prominent problem is that it has been difficult, if not impossible, to obtain square hysteresis loop characteristics from the deposition of a magnetic film directly upon the surface of the non-magnetic nickel. To obviate this difficulty, U.S. Pat No. 3,738,818 describes a process in which a thin layer of gold is deposited between the non-magnetic nickel and the magnetic cobalt layers. This process produces excellent results but introduces the complication of an additional step in the process as well as the added cost for the gold deposition.
The process of my invention eliminates the need for the intervening gold deposition step in the aforementioned patent and provides a magnetic coating with comparable characteristics by depositing directly upon the non-magnetic nickel layer a magnetic cobalt or cobalt nickel phosphorus film with thicknesses of 5 microinches or less down to approximately 1 microinch. At thickness less than 1 microinch it becomes difficult to maintain the unusually good loop shape characteristic of slightly thicker films. An important feature of the invention is that the electroless magnetic film bath possesses unusual stability with respect to short-term variations in the magnetic properties of the films produced therein, and the bath has been demonstrated to be capable of being used almost daily for several weeks without deterioration. The bath parameters affecting the magnetic properties are typically adjusted at the beginning of a work day and then remain stable throughout the day, provided, however, that the work load is such that there is no significant change in the hypophosphite level in the bath. This is in contrast to electroless magnetic plate baths containing ammonia as one of the constituents so that volatization of this constituent necessitates frequent replenishment. In those prior art baths containing the volatile ammonia, there are substantial changes in the coercive forces of the magnetic films produced in the bath because of the continual change in the pH of the bath. These pH changes produce problems of reproducibility of magnetic properties as well as the degradation in loop shape of the magnetic media produced during a single plating run.
The absence of volatile constituents, such as the ammonia used in prior art electroless plating processes, provides an additional feature of the invention. While the invention is primarily directed toward producing a magnetic layer for recording with good square loop properties at thicknesses approximately 5 microinches or less, the process can also be used to produce thicker magnetic structures with controlled high coercive force and equally good loop shapes by alternating thin layers of magnetic films with thin layers of electroless non-magnetic nickel. Heretofore, it has been extremely difficult to produce thick structures by this multi-layering technique because the volatization of the ammonia in the electroless bath caused rapid changes in the coervice force in the several films and resulted in creating poor hysteresis loop characteristics in the composite structure.